Processes for the formation of metallic layers over non-conducting substrates such as plastics and ceramics were known in the 1960's and consisted of applying a palladium catalyst to the substrate followed by electroless metallization.
The applications are naturally very diverse. Among them are the metallization of plastic articles such as automobile accessories, furniture, housewares, etc. or the manufacture of printed circuit boards and electromagnetic shields.
Typically, these procedures consist of the deposition of a thin layer of copper or electroless nickel over a previously catalyzed substrate, followed by reinforcement of the metallized layer through electrolytic plating. Deposition of only the electroless coating is generally referred to as the additive process whereas subsequent electrolytic plating is known as the semi-additive process.
There are today many variants, specifically for the manufacture of printed circuit boards, ("PCB's") that go from subtractive to completely additive methods. The subtractive method comprises removing a metal coating or layer from a non-metallic layer usually by etching the metal layer.
Several PCB's can be laminated to one another to form multilayer boards ("MLB's"). In MLB's the circuit of one board is connected to the circuit of one or more of the other boards in the multilayers. This is achieved by forming pads or circular areas of metal at a point or points on the conductive line or lines of the board. The pads may also be isolated from the conductive lines. The other board or boards that are to be connected are similarly provided with pads and in the laminating process the pads of the different boards are aligned over one another.
The MLB is then pressed and cured after which the pads of the MLB's are drilled to form through holes. The diameter of the drill is considerably less than the diameter of the pad, the ratio of diameters between the pad and the drill being about 2:1 or greater so that the overall structure comprises at a minimum a pad from one board aligned over a pad from another board with a through hole passing through them. Since the through hole in cross-section ideally presents a surface of alternating layers of the pads of the individual PCB's separated by the non-conductive base, an electrically conductive element has to be employed in the hole to form an electrical connection between the pads. This is done by a process known in the art as through hole plating (PTH).
PTH processes are also employed for connecting two metal conductive surfaces having a single non-conductive or dielectric board interposed between them for the formation of a PCB. Boards of this type and the formation of through holes in such boards are within the scope of the present invention and are intended to be included within the board definition of the PCB's as that term is used throughout the specification.
Before the PTH process can be undertaken, any "smear" in the hole must be removed.
After smear is removed, the through hole is plated. Electroless copper is employed as a PTH plating material. Standard electroless copper plating solutions known in the art are used for this purpose In order to promote the deposition of electroless copper on a non-conductive surface, the non-conductive surface is treated with a stannous chloride sensitizer solution followed by a super sensitizer solution of di-valent palladium chloride. The stannous chloride is oxidized to stannic chloride and the palladium chloride reduced to zero valent palladium.
A preferred method is to employ an activator comprising colloidal palladium containing stannous tin. Tin forms a protective colloid around the metallic palladium, and the solution implants a zero valent palladium site on the non-conductive surface for the purpose of initiating the deposition of the copper by chemical reduction. A post activator is then employed, generally an acid, to solubilize the protective colloid and expose the palladium.
The subsequently applied electroless copper coating solution contains metal ions, e.g. cupric ions and a reducing agent such as formaldehyde, which reduces the cupric ions in the solution to copper metal when in the presence of the palladium catalyst. The copper metal plates out on the surface of the through hole, making electrical contact with the walls of the metal pads in the through hole.
The board then goes to the "electrolytic" line where the copper deposit is reinforced, and an etch resist is applied (tin, tin-lead, gold, organic polymers, among others). Mask removal (stripping), etching, Sn/Pb reflow (if there is any) and final operations are the next steps.
After etching, additional processing may be employed including tin/lead removal, selective application of solder mask and selective application of solder through "hot air-levelling" or other similar methods.
In the metallization and image transfer processes, numerous variants have been tested with more or less success which include:
(a) Use of a colloidal copper based catalyst instead of palladium (LEA RONAL)
(b) Use of an ionic palladium catalyst without tin (SCHERING) (c) Use of an electroless copper solution that releases the "Accelerator" after catalysis with the classic mixed catalysts (SHIPLEY) (d) Hole metallization achieved in the electrolytic copper bath, through a modified preparation/catalysis in order to dispense with the electroless copper (EE-1 process of PCK, Morrissey et al. U.S. Pat. No. 4,683,036 and U.K. patent 2,123,036)
(e) Hole metallization achieved through colloidal carbon OLIN HUNT'S BLACK HOLE PROCESS)
However, most all the modifications and new procedures (the EE1 process being an exception) basically include the traditional approach which includes the basic steps of:
1 - Chemical Metallization PA1 2 - Image Transfer PA1 3 - Electrolytic Metallization PA1 1 - Image Transfer PA1 2 - Metallization
For many years, efforts have been made to develop ways in which the holes could be metallized after image transfer i.e. by a scheme of:
The difficulties of this process are two fold:
(a) The actual trend for the almost exclusive use of masks (plating-resists) developable in aqueous solution and removable in an alkaline aqueous environment, require that all the baths in the metallization sequence be acid (and not only those in the electrolytic metallization). This excludes the classic degreaser/conditioners as well as formaldehyde reduced electroless copper baths.
(b) The catalysts to be used must be selective in the sense that they must sensitize the surface of the holes, without sensitizing the surface of the mask.
None of the methods previously described allow such a selective process.
LEA RONAL, in collaboration with E.I. DU PONT DE NEMOURS & CO. INC., has developed a selective process restricted to semi-aqueous dry films, developable in solvents and removable in alkaline aqueous environment using electroless copper; however, the process was relatively complex, its application quite restrictive, and was thus put aside.
In 1989, the SCHLOTTER company introduced a selective process SLOTOPOSIT, characterized by using a preconditioning step (before image transfer) employing a gaseous phase containing SO.sub.3, a reduction step after catalysis and an electroless step with nickel at a pH or about 5.5 at a temperature of 40.degree. C. The process is compatible with all types of dry films, including those that are processed in an aqueous environment.